This invention is related to semiconductor devices. In particular, the invention is related to heterostructure field effect transistors (HFETs) that operate as switches.
High voltage semiconductor switches are key components in electronic circuits for power conversion. Examples of these applications include power supplies for electronic equipment, drives for electric motors, and inverters for solar cells.
A power switch has an on state that allows the device to conduct current, and an off state that prevents the device from conducting current. When in the on state, a power switch may conduct tens or hundreds of amperes while the voltage across the switch is less than one volt. When in the off state, the power switch typically must withstand hundreds or thousands of volts while conducting substantially zero current. The voltage that the device can withstand in the off state while conducting no more than a given small value of current is sometimes referred to as the breakdown voltage.
High voltage HFETs are fabricated to have, among other properties, a breakdown voltage that is predictable and stable. The fabrication of high voltage HFETs may include special structures of conductive components surrounded by an electrically insulating material. Such insulated conductors are sometimes called floating conductors because their electric potentials do not have a well-controlled relationship to a reference electric potential value under static conditions. The floating conductors may change their potentials in a controlled way under dynamic conditions.
Capacitive coupling between the conductive components within the electrically insulating material may allow the net total electric charges to distribute more or less uniformly within a region of the device in response to rapid changes in voltage at the terminals of the device. The distribution of charge during voltage transitions may prevent high local electric fields that can reduce the breakdown voltage. Use of charge distribution structures may also allow designers of the device to make accurate predictions of breakdown voltage before fabrication.
A perfect insulating material surrounding the conductive components of the charge distribution structure would prevent the conductive components from acquiring a static charge from the environment. Electric currents leaking through an imperfect insulating material can cause charge to accumulate on the conductive components of the charge distribution structure.
Electric fields from accumulated static charge can interfere with the ability of the charge distribution structure to prevent the high dynamic electric fields that reduce the breakdown voltage of the device. Since the accumulated static charge is the result of past electrical stress on the device, the breakdown voltage may depend on the device's history.
A solution is required to prevent the accumulation of static electric charge on floating conductors in semiconductor devices.